Various fine adjustments, or "correction factors", are built into the entire system. Here is an example in this sensor. If pressure in the compartment (1) -- with the throttle valve fully open -- is so high that it almost equals pressure in the vented compartment (2), the diaphragm (3) is affected. Usually this diaphragm is bent in by higher outside air pressure. Now it isn't, so the core can move into the gap even more, and sends an enrichment signal to the control box.